What’s wrong with saving the extra for left-over meals?
Put it into the fridge, or just freeze it and have it at another time.
I never throw out good food.
Jim
But there are times when one cannot, such as having a small portion left of a meal at a restaurant, but I am not returning home after the meal so have no means of keeping it cold.
Also, I have faced the situation with left-overs where more were reheated than should have been - and safe food practices prevent putting it back in the fridge or freezer after reheating.
You also have the conundrum of what to do with food about to expire - something I rarely face now, thanks to teenagers

but has happened when perishables are overbought (or too much is given to you by a friendly relative).
So to the OP - I would say it depends. If we’re talking about eating a few more bites to clean a plate, or finish something off, even when already satisfied, and then you reduce what you’d eat later in the day - then I’d say it’s perfectly acceptable.
If we’re talking about gorging oneself by eating much more than needed or desired just to prevent a waste of food in some situation - no, I would say that’s unacceptable because it’s damaging to your body.
People can take anything to extreme, so I guess what I’m trying to say is that common sense should prevail.
